A common modality in medicine is Photoplethysmograpy (PPG), where light from one or more LED's with possibly different wavelengths is transmitted though or reflected from a part of a body and detected by light receivers, such as photo diodes. The photo current is amplified and typically converted into digital signals, whereas the resulting light signal corresponds primarily to blood flow. This PPG-signal is a surrogate of blood pressure, but a translation into blood pressure values with a clinical acceptable accuracy is difficult. The signal is interfered with by noise, especially coming from changes of blood volume caused by vasomotor activity of the arteries. Thus, PPG-based wearable devices usually only measure heart rate, pulse oximetry, or are the second (mostly distal) point of measurement for propagation time methods.
Current wearable BP devices use a method where different sensors measure the propagation time of blood pulses from a proximal body part to a distal body part. This propagation time, which is also called “Pulse Transit Time” (PTT), “Pulse Arrival Time, (PAT)” or other commercially used names, is inversely related to blood pressure. Those PTT-methods and devices do not produce a true pulsatile continuous BP signal, as such devices can only measure one PTT episode per beat. Thus, the BP-signal from those devices cannot be used for further determination of hemodynamic parameters using biomedical algorithms like Pulse Contour Analysis and other pulse related methods. The BP signal measured from those devices needs further calibration with an intermittent upper-arm sphygmomanometer, also called non-invasive BP device (NBP), in a relatively short calibration interval. The calibration interval is dramatically shortened by vasomotoric influences, e.g. thermoregulation, stress or drugs.
Contrary to PTT-methods, WO 2013178475 describes an apparatus and method for the continuous, non-invasive determination of blood pressure (BP) by means of a photoplethysmographic (PPG) system, where the contact pressure of the PPG-system is modified according to the mean arterial pressure (mBP) in the finger. With this method, the true pulsatile BP in the finger or wrist can be obtained by adding the pulsatile PPG-signal PPG(t) multiplied with a calibration factor to the measured contact pressure CP(t) that equals to the mBP.
This method of continuous hemodynamic monitoring according to WO 2013178475 has several limits and cannot be used as a wearable sensor with clinical acceptable accuracy. For example, the system from WO 2013178475 does not allow for the measurement of absolute BP-values and needs persistent NBP calibration. For a wearable device, the permanent use of a NBP on the upper arm is not appropriate.
The system from WO 2013178475 is further influenced by orthostatic pressure difference between heart level and the actual finger level. For a correct use of WO2013178475, the finger or wrist has to be constantly kept on heart level, which is not appropriate for a wearable system.